This invention relates to measuring the concentration of nitrogen oxide compounds NO.sub.x in a gaseous mixture using a solid state sensing element comprising a semiconductor thin film. More particularly, this invention relates to determining the NO.sub.x concentration in a gaseous mixture by measuring the resistance of a tin oxide semiconductor thin film exposed to the gas. This invention is particularly useful for measuring NO.sub.x emissions in automotive exhaust gas.
Concern for air pollution has focused in a major part upon the emission of gaseous nitrogen oxide compounds, particularly from automobile internal combustion engines. Because nitrogen assumes a plurality of oxidation states, several nitrogen oxide compounds have been identified. As a group, these compounds are normally gases and are generally referred to by the symbol "NO.sub.x ", which relates to their common empirical formula. The most sigificant NO.sub.x are nitric oxide NO and nitrogen dioxide NO.sub.2. These two compounds are the principle NO.sub.x constituents found in automotive exhaust gas and in gases emitted from power plants and similar industrial sites.
When monitoring gaseous emissions, it is frequently desired to measure the gas composition at the site from which it emanates. Known methods for measuring the NO.sub.x concentration of a gas typically involve carefully controlled reactions or employe delicate and expensive instruments and, therefore, are not easily adapted for on-site measurements. For example, one widely accepted method involves chemiluminescence. Ozone is added to a gaseous sample and reacts with nitric oxide NO therein to produce an excited nitrogen dioxide NO.sub.2 molecule that subsequently emits light. Thus, the NO concentration is related to the intensity of the emitted light, which is measured by a photoelectric cell. Since chemiluminesence analysis is specific for NO, sample pretreatment is required to reduce initially present NO.sub.2 in order to measure the total NO.sub.x concentration. Because chemiluminescence analysis requires ozone reactant, carefully controlled reaction conditions, sample pretreatment, and delicate instruments, it is generally not suited for on-site NO.sub.x detection. For similar reasons, other known wet chemistry or instrumental methods useful for accurate laboratory NO.sub.x analysis are typically not suitable for measurements in the field.
Particular difficulty has been encountered in attempting to study and monitor NO.sub.x emissions of a moving automobile. Automobile engines operate over a wide range of conditions that are subject to continuous and rapid change. The hot exhaust gases contain varying concentrations of oxidizing species (NO.sub.x and O.sub.2) and also reducing species (CO and hydrocarbons). Also, on-board NO.sub.x detection must not significantly interfere with the flow of the exhaust gases. Therefore, what is needed for on-board NO.sub.x detection is a simple, direct method of accurately measuring the total NO.sub.x that provides continuous readings, responds rapidly to changes in the NO.sub.x concentration, and does not require the addition of reagents. The measurements must be made without sample pretreatment and without interference by other exhaust constituents. Any instruments employed must be durable, easily incorporated into the exhaust system, and have a low power consumption so as to be portable. Because no suitable on-board method has heretofore been available, automotive NO.sub.x studies have typically been limited to the dynamometer laboratory.
It has heretofore been known that the resistance of a thin film semiconductor exposed to a gaseous mixture is sensitive to the presence of certain species in the gas. Thus, thin film semiconductors have been used to measure the concentration of gaseous materials. For example, tin oxide thin films have been employed to measure reducing gases, such as hydrogen, hydrogen sulfide or a hydrocarbon, or oxidizing gases, such as oxygen. In view of this, it is totally unexpected that a highly selective tin oxide sensor could be adapted to measure the NO.sub.x concentration in the presence of these and other oxidizing and reducing compounds.
It is an object of this invention to provide a method for detecting NO.sub.x present in a gaseous mixture that does not require the addition of reagents to the mixture and does not employ delicate, expensive instruments. It is a further object of this invention to provide a method for accurately measuring the total NO.sub.x concentration of a gas, which measurement may be made without first treating the gas to change various NO.sub.x species to a single form or to remove other common gaseous constituents. The method of this invention is particularly useful for determining the total NO.sub.x concentration in automotive exhaust gases directly and without significant interference from O.sub.2, CO, hydrocarbons or other typical exhaust constituents.
It is a further object of my invention to provide a method for continuously monitoring the NO.sub.x concentration in a gas stream, which method responds quickly to changes in the gas NO.sub.x concentration and does not significantly interfere with the flow of the gas. More particularly, it is an object of my invention to directly and accurately measure the total NO.sub.x concentration in a gas by exposing the gas to a solid state sensor comprising a thin film semiconductor whose resistance is directly related to the presence of NO.sub.x.
It is a still further object of this invention to provide a solid state sensor comprising a thin film semiconductor whose resistance is sensitive to the presence of NO.sub.x but relatively insensitive to the presence of other oxidizing and reducing species, most notably O.sub.2, H.sub.2, CO, and hydrocarbons, whereby the resistance of the semiconductor provides an accurate means for calculating the NO.sub.x concentration. The sensor of this invention is durable, inexpensive and simple to manufacture and may be operated at low electrical power levels so as to be portable. In one aspect of this invention, the sensor is easily incorporated into an automotive exhaust system to provide continuous, on-board measurement of NO.sub.x emissions.